Summary: Viscoelastic Study Using an Atomic Force Microscope
Modified to Operate as a Nanorheometer
James W. G. Tyrrell and Phil Attard*
School of Chemistry F11, University of Sydney, New South Wales 2006, Australia
Received August 13, 2002. In Final Form: March 18, 2003
A commercial atomic force microscope (AFM) has been modified to operate as a nanorheometer using
only a PC-based hardware component and a custom software interface. Measurements on an agar gel
model substrate reveal a viscoelastic response that is well described by a recently proposed theory for
viscoelastic deformation. Both spectroscopic (harmonic response as a function of drive frequency) and
impulsive (relaxation following a step drive) measurements are made. The reciprocal of the frequency
correspondingtothemaximumphaseresponseisfoundtocoincidewiththesteprelaxationtimeinagreement
with theory. A relaxation time of 16.6 ms is obtained for the 4% agar gel, and 9.3 ms for the 3% agar gel.
Both theory and experiment give a phase lead which, although counterintuitive, has a physical explanation.
Material properties measured using the modified AFM show broad agreement with data generated using
a conventional parallel plate rheometer and illustrate a useful basis for the comparison of measurement
techniques.
Introduction
The atomic force microscope (AFM)1 is routinely em-
ployed to extract topographical information, with up to
subnanometer resolution,2 by profiling the sample surface